Fail-safe control rod drive system for nuclear reactor

ABSTRACT

A control rod drive system (CRDS) for use in a nuclear reactor. In one embodiment, the system generally includes a drive rod mechanically coupled to a control rod drive mechanism (CRDM) operable to linearly raise and lower the drive rod along a vertical axis, a rod cluster control assembly (RCCA) comprising a plurality of control rods insertable into a nuclear fuel core, and a drive rod extension (DRE) releasably coupled at opposing ends to the drive rod and RCCA. The CRDM includes an electromagnet which operates to couple the CRDM to DRE. In the event of a power loss or SCRAM, the CRDM may be configured to remotely uncouple the RCCA from the DRE without releasing or dropping the drive rod which remains engaged with the CRDM and in position.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/680,133 filed Aug. 6, 2012, which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to control rod drive systems for nuclearreactors, and more particularly to a fail-safe control rod drive system.

BACKGROUND OF THE INVENTION

A rod cluster control assembly (RCCA) comprises an array of tubularelements (“control rods”) containing neutron absorber “poison” connectedto a common support header for raising and lowering the control rodarray as a unit. The control rods in an RCCA are arrayed at a precisespacing, which ensures each rod is perfectly aligned with respectivecircular cavities in the fuel assemblies of the fuel core. The extent ofinsertion of the rod assembly into the fuel core is controlled by thedevice referred to as a control rod drive mechanism (CRDM), which is asubcomponent of the control rod drive system (CRDS).

In typical pressurized light water reactors (PLWRs), the CRDM isoperated from the top of the reactor head which is approximately 15 to20 feet above the top of the nuclear fuel core. However, in certain newreactor systems, the height of the reactor head may be many timesgreater above the top of the fuel core. For example, in the HI-SMUR™SMR-160 from Holtec International, the RCCAs may require operation froma distance of over 60 feet, which using the present existing technology,would require the drive rod (DR) which is normally supplied withexisting CRDM to be in excess of 60 feet long. DRs with such a longlength, however, would be impractical for the following reasons:

Removing drive rods from the reactor vessel would require an inordinateamount of crane head room;

Performing routine maintenance would require a large laydown area;

The weight of the drive rod becomes so large due to the increasedlength, that during a SCRAM (emergency shutdown procedure of the reactorin which control rod are quickly inserted into the fuel core to suppressthe nuclear reaction), the top nozzle of the fuel assembly risksbecoming damaged from the weight of the falling RCCA as well as the ESA;

During a SCRAM, the drive rod is at risk of being damaged because of theinertia load, which is magnified in the CRDM which utilizes a lead screwfor the drive rod; and

Manufacture of drive rod becomes difficult thereby increasing the costto fabricate the CRDS.

Another problem is presented by the location of the CRDM. Contemporarycommercial technology requires the CRDM to be installed External to theReactor Vessel. This presents major concerns with regards to theoperational safety of the CRDS. With presently available technologyshould a failure of the pressure retaining portion of the CRDM occur thepressure differential between the inside of the reactor vessel and theatmosphere external to the reactor vessel would subsequently cause theCRDM drive rod to be ejected from the reactor. This in turn could causea spike in the reactivity of the reactor core, since the drive rod ismechanically connected to the RCCA in the current state-of-the-arttechnology.

One solution would be to locate CRDM within the reactor vessel. However,this would pose several technical challenges. First, control rod drivemechanisms are complex electromechanical devices. Exposing these to thehigh pressure and temperature environment inside the reactor vessel cancause the mechanism to fail prematurely. Second, placing the control roddrive mechanism inside the reactor vessel presents possibly structuralproblems since the mechanism is also subject to flow induced vibration.Accordingly, although this approach would solve the long drive rodproblem, it is undesirable for the foregoing reasons.

An improved control rod drive system is desired.

SUMMARY OF THE INVENTION

The present invention provides a control rod drive system (CRDS) thatovercomes the foregoing problems and yields a number of additionalbenefits, which will be readily discerned from the description whichfollows. The present invention may be beneficially used for nuclearreactor vessel designs of a high head design described above (e.g. topof the reactor head located at a vertical distance greater thanapproximately 15 to 20 feet above the top of the nuclear fuel core), buthas broader application as well to virtually any reactor vessel design.

In one configuration, a control rod drive system (CRDS) generallyincludes a drive rod mechanically coupled to a control rod drivemechanism operable to linearly raise and lower the drive rod along avertical axis, a rod cluster control assembly (RCCA) comprising aplurality of control rods positioned proximate to and insertable into anuclear fuel core, and a drive rod extension (DRE) releasably engagedbetween the drive rod and RCCA. The CRDS is remotely operable toselectively couple and uncouple the DRE from the RCCA and drive rod. TheCRDM includes an electromagnet which releas ably couples the CRDM toDRE. This arrangement contrasts to known CRDSs in which the drive rod isdirectly coupled to the RCCA, which is unsuitable in situationsrequiring drive rods with excessively long lengths (e.g. greater than15-20 feet). In the event of a power loss or SCRAM, the CRDM may beconfigured to remotely uncouple the RCCA from the DRE without releasingor dropping the drive rod which remains engaged with the CRDM and inaxial position. Advantageously, this protects the integrity of the CRDMand eliminates potential problems with known designs caused by droppingthe drive rod which may damage equipment, as described above. Thepresent DRE includes unique features providing the remote coupling anduncoupling functionality, and failsafe operation in the event of a powerloss or SCRAM, as further described herein.

According to one exemplary embodiment of the present invention, acontrol rod drive system for a nuclear reactor vessel includes: avertically oriented drive rod mechanically coupled to a control roddrive mechanism operable to raise and lower the drive rod through aplurality of axial positions; a rod cluster control assembly comprisinga plurality of control rods configured for removable insertion into anuclear fuel core; a drive rod extension extending axially between therod cluster control assembly and the drive rod, the drive rod extensionhaving a bottom end releasably coupled to the rod cluster controlassembly; and a drive rod extension grapple assembly connected to thedrive rod, the grapple assembly releasably coupled to a top end of thedrive rod extension. Raising and lowering the drive rod raises andlowers the rod cluster control assembly. In one embodiment, the grappleassembly includes an electromagnet which magnetically couples the driverod extension to the grapple assembly when the electromagnet isenergized and uncouples the drive rod extension from the grappleassembly when the electromagnet is de-energized.

According to another exemplary embodiment, a control rod drive systemfor a nuclear reactor vessel includes: a control rod drive mechanismmounted externally to the reactor vessel; a drive rod mechanicallycoupled to the control rod drive mechanism and extending through thereactor vessel into an interior cavity of the reactor vessel holding anuclear fuel core, the control rod drive mechanism operable to raise andlower the drive rod through a plurality of vertical axial positions; agrapple assembly connected to the drive rod in the interior cavity ofthe reactor vessel and movable with the drive rod; an electromagnetmounted in the grapple assembly; a rod cluster control assemblycomprising a plurality of control rods configured for removableinsertion into the nuclear fuel core; and a drive rod extensionextending axially between the rod cluster control assembly and thegrapple assembly. The drive rod extension includes: an axially extendingactuator shaft having a top end including a magnetic block configured toreleas ably engage the electromagnet of the grapple assembly and abottom end configured to releasably engage the rod cluster controlassembly; and a lifting head sleeve including a diametrically enlargedlifting head, the lifting head sleeve slidably receiving the actuatingrod therethrough for axial upward and downward movement. Theelectromagnet is operable to magnetically couple the actuating shaft tothe grapple assembly at the top of the drive rod extension when theelectromagnet is energized and uncouple the actuating shaft from the rodcluster control assembly at the bottom of the drive rod extension whenthe electromagnet is de-energized. Raising the actuator shaft when theelectromagnet is energized couples the actuator shaft to the rod clustercontrol assembly and de-energizing the electromagnet lowers anduncouples the actuating shaft from the rod cluster control assembly.

According to another exemplary embodiment, a control rod drive systemfor a nuclear reactor vessel includes: a reactor vessel having a tophead and an interior cavity; a nuclear fuel core supported in theinterior cavity of the reactor vessel; a rod cluster control assemblycomprising a plurality of control rods configured for removableinsertion into the nuclear fuel core; a control rod drive mechanismmounted externally to the reactor vessel above the top head; a drive rodmechanically coupled to the control rod drive mechanism and extendingthrough the top head of reactor vessel into the interior cavity, thecontrol rod drive mechanism operable to raise and lower the drive rodthrough a plurality of vertical axial positions; a grapple assemblyconnected to the drive rod inside the interior cavity of the reactorvessel and movable with the drive rod, the grapple assembly including anelectromagnet; a drive rod extension extending axially between the rodcluster control assembly and the grapple assembly, the drive rodextension including a bottom end releasably coupled to the rod clustercontrol assembly and a top end releasably coupled to the grappleassembly via the electromagnet; and a longitudinally-extending drive rodextension support structure mounted in the reactor vessel above thenuclear fuel core, the support structure including a plurality ofvertically-oriented guide tubes at least one of which is configured toslidably receive the drive rod extension therein for axial upward anddownward movement. The electromagnet is operable to magnetically couplethe drive rod extension to the grapple assembly when the electromagnetis energized and uncouple the drive rod extension from the grappleassembly when the electromagnet is de-energized. De-energizing theelectromagnet drops and uncouples the drive rod extension from the rodcluster control assembly remotely at the bottom of the drive rodextension.

An exemplary method for coupling a control rod drive mechanism to a rodcluster control assembly in a nuclear reactor vessel is provided. Themethod includes the steps of: providing: a reactor vessel having a tophead and an interior cavity; a nuclear fuel core supported in theinterior cavity; a rod cluster control assembly positioned at a top ofthe fuel core and comprising a plurality of control rods configured forremovable insertion the fuel core; a control rod drive mechanism mountedexternally above the reactor vessel; a drive rod assembly including adrive rod mechanically coupled to the control rod drive mechanism andextending into the interior cavity of the reactor vessel, and a grappleassembly disposed on an end of the drive rod and including anelectromagnet. The method further includes lowering the drive rodassembly; contacting the drive rod assembly with a top end of a driverod extension extending vertically between the rod cluster controlassembly and the top head of the reactor vessel, a bottom end of thedrive rod extension contacting the rod cluster control assembly in anon-locking manner; engergizing the electromagnet to magnetically couplethe drive rod assembly with the drive rod extension; raising the driverod assembly by a first vertical distance; locking the bottom end of thedrive rod extension with the rod cluster control assembly, whereinraising and lowering the drive rod assembly with the control rod drivemechanism raises and lowers the rod cluster control assembly forcontrolling the reactivity within the fuel core.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the exemplary embodiments of the present invention willbe described with reference to the following drawings, where likeelements are labeled similarly, and in which:

FIG. 1 is a side elevation diagrammatic view of the upper head portionof a nuclear reactor vessel with an exemplary embodiment of control roddrive system according to the present disclosure;

FIG. 2 is a side elevation diagrammatic view of the full reactor vesselof FIG. 1;

FIG. 3 is a perspective view of a drive rod extension grapple assembly;

FIG. 4 is a side cross-sectional view thereof;

FIG. 5 is an perspective view of a drive rod extension supportstructure;

FIG. 5A is an enlarged detail VA taken from FIG. 5;

FIG. 5B is an enlarged detail VB taken from FIG. 5;

FIG. 6 is a side cross-sectional view of the drive rod extension supportstructure;

FIG. 6A is an enlarged detail VIA taken from FIG. 6;

FIG. 7 is an perspective view of a drive rod extension support structurewith drive rod extensions mounted therein;

FIG. 8 is a side cross-sectional view of a drive rod extension supportstructure mounted above a nuclear fuel core;

FIG. 8A is an enlarged detail VIIIA taken from FIG. 8;

FIG. 8B is an enlarged detail VIIIB taken from FIG. 8;

FIG. 9 is a perspective view of a drive rod extension;

FIG. 10A is a side cross-sectional view of the upper portion of thedrive rod extension shown in FIG. 9;

FIG. 10B is a side cross-sectional view of the lower portion of thedrive rod extension shown in FIG. 9;

FIG. 11A is a side cross-sectional view of the top end of the drive rodextension and drive rod extension grapple assembly in an uncoupledposition;

FIG. 11B is a side cross-sectional view of the bottom end of the driverod extension and rod cluster control assembly in an uncoupled positioncorresponding to the position of the grapple assembly shown in FIG. 11A;

FIGS. 12A and 12B are sequential side cross-sectional views of the topend of the drive rod extension and drive rod extension grapple assemblyduring the drive rod extension and drive rod extension grapple assemblycoupling process;

FIG. 13 is a side cross-sectional view of the top end of the drive rodextension and drive rod extension grapple assembly in a coupledposition;

FIG. 14A is a side cross-sectional view of the top end of the drive rodextension and drive rod extension grapple assembly in a higher coupledposition than FIG. 13 showing a lifting head sleeve of the drive rodextension disengaged from a retaining collar in the drive rod extensionsupport structure;

FIG. 14B is a side cross-sectional view of the bottom end of the driverod extension and rod cluster control assembly in a fully coupled andlocked position;

FIG. 15A is a side cross-sectional view of the top end of the drive rodextension and drive rod extension grapple assembly shown in an uncoupledposition;

FIG. 15B is a side cross-sectional view of the bottom end of the driverod extension and rod cluster control assembly in an uncoupled andunlocked position corresponding to the position of the grapple assemblyshown in FIG. 15A;

FIG. 16A is a side cross-sectional view of the top end of the drive rodextension and drive rod extension grapple assembly in a coupled positionwith the grapple assembly in a lowermost position on the drive rodextension;

FIG. 16B is a side cross-sectional view of the bottom end of the driverod extension and rod cluster control assembly with the bottom end ofthe drive rod extension in a lowermost position in the rod clustercontrol assembly corresponding to the position of the grapple assemblyshown in FIG. 16A;

FIGS. 17A-C show sequential steps in a process for uncoupling anddismounting the top end of the drive rod extension from the drive rodextension grapple assembly; and

FIG. 18 shows a diagrammatic illustration of an exemplary control roddrive mechanism.

All drawings are schematic and not necessarily to scale. Parts given areference numerical designation in one figure may be considered to bethe same parts where they appear in other figures without a numericaldesignation for brevity unless specifically labeled with a differentpart number and described herein. In addition, a reference to a singlefigure number prefix (e.g. FIG. 10) which comprises multiple figures ofthe same prefix number distinguished by different alphabetical suffixes(e.g. FIGS. 10A and 10B) shall be construed as a general reference toall figures sharing that same prefix number.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The features and benefits of the invention are illustrated and describedherein by reference to exemplary embodiments. This description ofexemplary embodiments is intended to be read in connection with theaccompanying drawings, which are to be considered part of the entirewritten description. In the description of embodiments disclosed herein,any reference to direction or orientation is merely intended forconvenience of description and is not intended in any way to limit thescope of the present invention. Relative terms such as “lower,” “upper,”“horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and“bottom” as well as derivative thereof (e.g., “horizontally,”“downwardly,” “upwardly,” etc.) should be construed to refer to theorientation as then described or as shown in the drawing underdiscussion. These relative terms are for convenience of description onlyand do not require that the apparatus be constructed or operated in aparticular orientation. Terms such as “attached,” “affixed,”“connected,” “coupled,” “interconnected,” and similar refer to arelationship wherein structures are secured or attached to one anothereither directly or indirectly through intervening structures, as well asboth movable or rigid attachments or relationships, unless expresslydescribed otherwise. Accordingly, the disclosure expressly should not belimited to such exemplary embodiments illustrating some possiblenon-limiting combination of features that may exist alone or in othercombinations of features.

System Component Definitions

In one non-limiting example to provide an overview, a control rod drivesystem according to the present disclosure may generally include thefollowing major assemblies defined below in summary fashion and furtherdescribed herein in greater detail:

Rod ejection protection device (REPD)—a hydraulically-actuatedmechanically-returned collet which engages the drive rod of the CRDM andprevents the drive rod from moving in position in the event of a failureof the CRDS.

Control rod drive mechanism (CRDM)—An electro mechanical device used tocontrol the position of the Control Rods located in the reactor core

Drive rod (DR)—A shaft that passes through the CRDM into the reactorvessel through the reactor vessel nozzle and is attached to the DREGA.

Drive rod extension grapple assembly (DREGA)—An assembly that is used toconnect the DR to the DRE. This assembly also contains an electromagnetwhich, when energized and de-energized, engages and disengages the DREwith the RCCA respectively.

Drive rod extension support structure (DRESS)—a support structuredesigned to hold and guide the DREs. In one illustrative embodiment, forexample without limitation, the DRESS may include thirty seven guidetubes. The guide tubes may be perforated to allow for water circulation(e.g. primary coolant) therethrough. Retaining collars (located at thetop of the DRESS) may hold spring loaded retention devices. Thesedevices attach to the DRE lifting head sleeve. Their purpose is toprevent the guide DRE from being removed from the DRESS inadvertentlyduring reactor vessel head removal. The DRESS provides lateral andseismic restraint of the DREs.

Drive rod extension (DRE)—A device that is connected to the DR by meansof the DREGA which extends the reach of the DR to engage the RCCAlocated below.

FIGS. 1 and 2 depict an exemplary embodiment of a control rod drivesystem 100. The control rod drive system 100 is shown installed on areactor vessel 110 which includes a longitudinally-extending andelongated cylindrical shell 111 defining a vertical axis, bottom head112, and top head 113. In one embodiment, the top head 113 may beremovable form the shell 111 such as via a bolted flange joint or otherform of detachable mounting. The reactor vessel defines an interiorcavity 114 which holds a core support structure 115 configured tosupport a nuclear fuel core 116. In one embodiment, the core supportstructure 115 may be in the form of a tubular riser pipe 119 whichconveys primary coolant flowing in an annular space 118 between theriser pipe 119 and shell 111 upwards through the fuel core 116 andoutwards through a flow nozzle 117 fluidly coupled to a steam generatorfor generating steam. The primary coolant is heated by flow upwardsthrough the fuel core 116. In one embodiment, the fuel core 116 may inthe form of a self-supporting fuel cartridge such as the SMR-160 unitaryfuel cartridge available from Holtec International which is insertableinto the core support structure 115. As will be well known to thoseskilled in the art without undue elaboration, a typical nuclear reactorcore in a light water reactor comprises tightly packed fuel assemblies700 (also referred to as fuel bundles) as further shown in FIG. 8B. Eachfuel assembly 700 is an assemblage of bundled fuel rods 702 which aresealed hollow cylindrical metal tubes (e.g. stainless steel or zirconiumalloy) packed with enriched uranium fuel pellets and integral burnablepoisons arranged in an engineered pattern to facilitate as uniform aburning profile of the fuel as possible (in both axial and crosssectional/transverse directions). Multiple longitudinally-extendingcavities are formed within each fuel assembly 700 for insertion of thecontrol rods 504 into the fuel core in the usual manner, such as throughthe top nozzles boxes 704 mounted atop each fuel assembly 700 which aredisposed proximate to the bottom of the drive rod extension supportstructure (DRESS) 160 and accessible to the RCCAs 500. Numerousvariations in the arrangement are possible.

It will be appreciated that numerous variations are possible in thearrangement of components within the reactor vessel 110; the foregoingarrangement described representing only one possible exemplaryembodiment. Accordingly, the invention is not limited in this regard tothe embodiment described herein.

As shown in FIG. 2, reactor vessel 110 may be considered a high headreactor vessel design in which the fuel core 116 is disposed near thebottom head 112 of the vessel within the core support structure 115riser pipe. The distance between the top of the fuel core and top head113 of the reactor vessel may exceed the usual 15-20 feet distance intypical pressurized light water reactors (PLWRs).

The reactor vessel 110 may be made of any suitable metal, such as forexample without limitation steels such as stainless steel for corrosionresistance.

With continuing reference to FIGS. 1 and 2, control rod drive system 100includes drive rod (DR) 130, drive rod extension (DRE) 400, drive rodextension support structure (DRESS) 160, drive rod extension grappleassembly (DREGA) 200, control rod drive mechanism (CRDM) 300, and rodejection protection device (REPD) 140. Other than the DRESS 160 and fuelcore 116 for which a single assembly of each may be provided for areactor vessel 110, the control rod drive system (CRDS) 100 may actuallyinclude a plurality of the foregoing remaining components eachassociated with providing a lifting mechanism for raising/lowering oneof the plurality of rod cluster control assemblies (RCCA) 500 (see, e.g.FIG. 11B) provided with the reactor vessel 110. Accordingly, there mayin fact be a plurality of the component assemblies shown in FIGS. 1 and2 although only a single CRDM 300 rod drive mechanism 300 and associatedlifting components are shown for clarity of description. In oneexemplary embodiment, for illustration, a reactor vessel 110installation of a small modular reactor design may include approximately37 CRDMs 300 and associated DREs 400. The invention is not limited toany particular number of CRDMs or other components.

Control rod drive mechanisms 300 may each be housed in a structuralenclosure 302 mounted to top head 113 of reactor vessel 100 forprotection of the drive mechanism. The function of this enclosurestructure includes to provide lateral and seismic support of the CRDMs300, protect the CRDMs from projectile or missile generated within theprimary containment structure (not shown) which encloses the reactorvessel 110, protect the CRDMs from potential drops of equipment from theoverhead crane, provide a means of lifting the reactor vessel head, andprovide a mounting location for the REPD 140 which may be mounted on topof enclosure 302 in one embodiment. The CRDM enclosures 302 may beattached to the reactor vessel top head 113 by any suitable means, suchas without limitation welding.

In one embodiment, the top head 113 of reactor vessel 110 may include aflanged nozzle 304 configured to receive a bottom mounting flange 306 oncontrol rod drive mechanism 300 for coupling and supporting the drivemechanism from the reactor vessel head. The bottom mounting flange 306may be detachably coupled to the flanged nozzle 304 with fasteners (e.g.bolts and nuts) to allow the control rod drive mechanism 300 to beremoved for maintenance or replacement. The drive rod 130 extendsvertically downwards through the rod ejection protection device 140, topof the enclosure 302, control rod drive mechanism 300, and furtherthrough the flanged nozzle 304 into the top portion of reactor vesselbeneath top head 113 as shown in FIGS. 1 and 2. A set of seals may beprovided with the drive rod 130 at the flanged nozzle 304 to preventleakage of reactor coolant from the reactor vessel along the drive rodduring operation. The bottom end of the drive rod 130 is coupled to thedrive rod extension grapple assembly (DREGA) 200, as further describedherein.

Control rod drive mechanism (CRDM) 300 may be any type of commerciallyavailable electro-mechanical drive operable to lower/raise the drive rod130 (and in turn DREGA 200 attached to the drive rod). As onenon-limiting example diagrammatically illustrated in FIG. 18, a CRDM 300of one type may have a drive assembly 600 generally utilizing a motordrive to rotate a lead screw 604 formed on the drive rod 130. Such drivemechanisms for drive rods are well known to those skilled in the art. Inone arrangement, as shown, the electric drive motor 610 may be axiallyoffset from the drive rod 130 and rotates a worm 608 (i.e. worm gear)arranged transversely to the drive rod. The worm 608 in turn rotates aring gear 606 rigidly affixed to a ball collar or nut or collar 602having ball bearings 612 engaged with the lead screw 604 on the driverod 130. Rotating the ring gear 606 in opposing directions using themotor drive 610 which operates to rotate the worm 608 in opposingrotational directions alternatingly axially raises or lowers the driverod 130 in a controlled manner. In other possible arrangements, the ballnut or collar may be directly coupled to the drive motor which may bearranged axially in line with the drive rod. In either of the foregoingarrangements, the CRDM rotates the ball nut or collar which axiallyadvances or retracts the drive rod via the lead screw. Numerousvariations of CRDMs using drive rod lead screws are possible. CRDMs arecommercially available from a number of manufacturers, including forexample General Atomics of San Diego, Calif. CRDMs are further describedin U.S. Pat. No. 5,999,583 and U.S. Patent Application Publication2010/0316177, which are incorporated herein by reference in theirentireties.

FIGS. 3 and 4 show drive rod extension grapple assembly (DREGA) 200 ingreater detail. DREGA 200 includes a cylindrical grapple body 202 havingsidewalls 232 defining an interior chamber 212, an open top 224, and adownwardly open bottom 226. Top 224 may be closed by a removable topplate 204 in one embodiment which is attached to the top annular face ofgrapple body 202 via a plurality of circumferentially spaced fasteners206. The open bottom 226 allows an upper portion of drive rod extension400 to be inserted therein, as further described herein. Anelectromagnet 228 is disposed in chamber 212 which is engageable with amagnetic block 402 of drive rod extension 400 (see, e.g. FIG. 9). In oneembodiment, electromagnet 228 may be mounted at the top end of chamber212 and affixed to the underside of top plate 204 by one or morefasteners 208. Other variations for mounting electromagnet 228 arepossible.

With continuing reference to FIGS. 3 and 4, drive rod extension grappleassembly (DREGA) 200 further includes plurality of circumferentiallyspaced and radially movable lifting pins 216. Lifting pins 216 may beoriented horizontally in one embodiment and are operable to projectradially inwards into chamber 212 towards the vertical centerline ofgrapple body 202 through corresponding circumferentially spaced openings214 formed through the body. The lifting pins 216 are radially movablebetween a projected position (shown in FIG. 4) extending partially intothe chamber 212 and a retracted position withdrawn from the chamber.Lifting pins 216 may each be biased inwards towards the projectedposition via a suitably configured lift spring 218 having an end whichengages an outward facing open socket formed in each pin as shown.

In one embodiment, lifting pins 216 may be movably disposed in anannular shaped housing 222 which extends radially outwards from grapplebody 202. Housing 222 includes a plurality of circumferentially spacedbores 230 having a circular cross section configured to slidably receivelifting pins 216 therein. Bores 230 may extend radially completelythrough the housing 222 and sidewalls 232 of grapple body 202communicating with openings 214. Each bore 230 includes a lifting pin216 and associated spring 218. The lifting pins 216 may include astepped shoulder 234 which engages a complementary configured steppedportion of the bore 230 to prevent the lifting pins from being ejectedby the spring 218 completely through holes 214 into the chamber 212 ofthe grapple body 202. In one embodiment, the exterior opening in eachbore 230 may be closed off by a removable cap 220 which threadablyengages the annular housing 222. The caps 220 each have an interiorsurface which may engage one end of spring 218. In one embodiment, theannular housing 222 may be threaded along an exterior portionsurrounding each bore 230 and the caps 220 may threadably engage thesethreaded bore surfaces. Other suitable arrangements of mounting caps 220to close bores 230 may be used.

The drive rod extension grapple assembly (DREGA) 200 may be mounted tothe bottom end of the drive rod 130 by any suitable means. For example,without limitation, drive rod 130 may be threadably coupled directly toDREGA 200 via a threaded socket formed in the top plate 204 andthreading the bottom end of the drive rod, via mounting brackets andfasteners, welding, or other suitable mechanical mounting techniquesused in the art. Preferably, in certain embodiments, DREGA 200 isrigidly mounted to the drive rod 130.

In one embodiment, cylindrical grapple body 202 may have a maximumoutside diameter larger than the interior diameter of the flanged nozzle304 so that the DREGA A cannot be inserted or retracted through thenozzle. In such an arrangement, the DREGA 200 is connected to the end ofthe drive rod 130 beneath the top head 113 of the reactor vessel 110.Other suitable arrangements are possible.

FIGS. 5-8 (including all alphabetical subparts) depict the drive rodextension support structure (DRESS) 160. DRESS 160 is a verticallyelongated structure which includes a plurality of upper guide tubes 161and lower guide tubes 162 circumscribed by an open lattice outer supportframe 163 having a cylindrical shape to complement the shape of theriser pipe 119 in which the DRESS may be inserted from the top. The openstructure reduces the weight of the support frame 163 while providingstructural strength. In one exemplary embodiment, without limitation,the outer support frame 163 may have an X-shaped lattice formed bydiagonal supports 164 arranged in an X-pattern and enlarged junctionplates 165 formed at the intersection of the diagonal supports. Othersuitable open or closed structures are possible for support frame 163.

The upper and lower guide tubes 161, 162 may be intermittently supportedalong their lengths by axially spaced apart horizontal supports 166. Ahorizontal support 166 is provided at the top 166 a and bottom 166 b ofDRESS 160. In one exemplary embodiment, the supports 166 may be spacedaxially apart at approximately 5-6 feet intervals along the longitudinallength of the guide tubes 161, 162. Other appropriate axial spacing maybe used.

In one embodiment, the horizontal supports 166 may be comprised ofinterconnected lateral grid plates 171 extending between adjacent guidetubes 161, 162. The outermost supports 166 may be attached at their endsto an annular shaped peripheral rim 169 which may be attached to theinterior surface of the cylindrical outer support frame 163, such as atthe junction plates 165 and/or along horizontal arcuately shaped strapmembers 167 connected between junction plates. In one embodiment, thehorizontal supports 166 may be welded to the outer support frame;however, other suitable attachment methods may be used instead of or inaddition to welding such as fasteners.

In one embodiment, the uppermost horizontal support 166 may include anarray of laterally spaced circular retaining collars 170 mounted ontothe top ends of each upper guide tube 161. This forms a grid array ofretaining collars 170 having a pattern or layout in top plan view whichmatches the horizontal pattern or layout of the upper guide tubes 161.The retaining collars 170 each have a central opening configured toreceive a respective upper guide tube therein. The retaining collars170, located at the top of the drive rod extension support structure(DRESS) 160, may include spring loaded retention devices in the form ofradially movable retaining pins 172 spaced circumferentially around theretaining collars (see, e.g. FIGS. 5A and 11A). The retaining pins 172may be horizontally oriented and movable to be retracted from orprojected into the central hole of the retaining collar 170. As notedabove the retaining pins 172 engage the DRE lifting head sleeve 408 (seealso FIGS. 10 and 11). One of their purposes is to prevent the guide DRE400 from being removed from the DRESS 160 inadvertently during reactorvessel head removal.

The upper guide tubes 161 have a diameter selected to allow the driverod extension (DRE) 400 to be axially inserted completely through theguide tube in one embodiment. This allows raising and lowering of theDREs 400 by the control rod drive mechanism (CRDM) 300. Each of thelower guide tubes 162 may have a larger diameter than the upper guidetubes 161. The lower guide tubes 162 have a diameter selected to allowthe entire control rod support plate 502 of the rod cluster controlassembly (RCCA) 500 (shown in FIG. 11B) to be raised and lowered withinthe lower guide tubes for inserting and retracting the control rods 504into and from the fuel core 116. The control rod support plate 502 has alarger diameter than the widest component of the DRE 400 in the presentexemplary embodiment, thereby necessitating a larger diameter for thelower guide tubes 162 than the upper guide tubes 161.

In one embodiment, guide tube transition fittings 168 may be used tocouple the lower ends of each upper smaller diameter upper guide tube161 to a corresponding concentrically aligned lower guide tube 162. Inone embodiment, the transition fittings 168 may be frusto-conical shapedas best shown in FIGS. 5B and 6A and have an open structure comprised ofaxially spaced apart upper and lower rings 168 a, 168 b each attachedrespectively to an upper and lower guide tube 161, 162. Accordingly, thelower rings 168 b have a larger diameter than the upper rings 168 a inthis embodiment. The rings 168 a, 168 b may be joined to form astructural unit by angled and vertically extending struts 168 cextending between the rings. In other embodiments, the guide tubetransition fittings 168 may be closed. Other suitable configurations ofguide tube transition fittings 168 are possible includingnon-frusto-conical shapes. The guide tube transition fittings 168 helpmaintain axial alignment between the upper and lower guide tubes 161,162. The guide tubes 161, 162 in turn help maintain axial alignment ofthe control rods with respective corresponding cavities in the fuel core118 for insertion or retraction of the rods to control the nuclearreaction rate in various portions of the core. Other suitableconfigurations of transition fitting, however, may be used and numerousvariations are possible.

In some embodiments, the upper and lower guide tubes 161, 162 may eachinclude a plurality of holes or perforations along their respectivelengths as shown in FIGS. 5-8 which allow the primary coolant to flowinside the guide tubes within the riser pipe 119. The holes orperforations may be distributed both circumferentially andlongitudinally around each guide tube 161, 162 in a suitable pattern.

Referring to FIGS. 2 and 8, the drive rod extension support structure(DRESS) 160 may be mounted inside the upper portion of riser pipe 119proximate to the top of the fuel core 116. This allows the loweroperating ends of each drive rod extensions (DREs) 400 which may becoupled and uncoupled from the rod cluster control assembly (RCCA) 500to be in proper position for inserting or retracting the control rods504 into/from the fuel core 116 for controlling the nuclear reactionrates in parts or all of the fuel core, as further described herein.

FIGS. 9 and 10 show the drive rod extension (DRE) 400 in greater detail.Each DRE 400 is intermediate link which operably couples a drive rod 130at top end 401 of the DRE to a corresponding rod cluster controlassembly (RCCA) 500 at bottom end 403 of the DRE. DRE 400 includes aninner actuator shaft 404 which is disposed inside an outer actuator tube406 and a lifting head sleeve 408. Actuator shaft 404 extendslongitudinally for substantially the entire length of the DRE 400 andmay be a single unitary structure in some embodiments.

In one embodiment, lifting head sleeve 408 is positioned at an upperportion of the DRE above the top of the drive rod extension supportstructure (DRESS) 160. Lifting head sleeve 408 has a bottom end 421 anda top end 412 that abuts a lower surface 414 of a diametrically enlargedlifting head 410. Axially spaced between ends 412 and 421 is an annularstop flange 416 extending radially outwards from lifting head sleeve408. The stop flange 416 is configured to engage an axially movablebobbin 430 which is slidable on lifting head sleeve 408 and defines alower travel stop for the bobbin. Stop flange 416 may be furtherarranged to engage the top of retaining collar 170 to limit theinsertion depth of the lifting head sleeve into the upper guide tube 161(see also FIG. 11A).

Lifting head sleeve 408 may further include a stepped portion 420 whichdefines a downward facing surface which abuts a top end 422 of actuatortube 406. In one embodiment, the bottom end 421 of lifting head sleeve408 may be sized to be inserted into the open top end 422 of actuatortube 406.

An axial portion of lifting head sleeve 408 disposed between stop flange416 and stepped portion 420 defines a recessed annular seating surface423 configured to removably receive and engage spring biased retainingpins 172 of retaining collar 170 which is initially positioned aroundthe lifting head sleeve at this location (see also FIGS. 5A and 11A).

With continuing reference to FIGS. 9 and 10, bobbin 430 includes anoutward-upward facing angled upper bearing surface 432 and an opposingoutward-downward facing angled lower bearing surface 434 which meet at acircumferentially extending apex A. Lower bearing surface 434 isselectively engageable with 216 of drive rod extension grapple assembly(DREGA) 200. Upper bearing surface 432 is selectively engageable withlifting head 410. The functionality of these bearing surfaces will befurther described herein.

Lifting head 410 may be an annular generally inverted cup-shaped memberin some embodiments. Lifting head includes an annular outward-upwardfacing angled upper bearing surface 424 and opposing annularinward-downward facing angled lower bearing surface 414. Bearing surface414 defines a downwardly open cavity 426 which is configured to receiveand complement the configuration of bobbin upper bearing surface 432. Aportion of lower bearing surface 414 is engaged by top end 412 oflifting head sleeve 408 to maintain the axial position of the liftinghead 410. Lifting head 410 has a larger diameter than the top end 412 oflifting head sleeve 408.

DRE 400 may further include a drive extension spring 462 having a bottomend engaging a top surface 427 of lifting head 410. Spring 462 isarranged concentrically around actuator shaft 404 and may be a helicalcoil spring in some embodiments. In one embodiment, a hollow andcylindrically-shaped spring retainer 460 may be provided which holdsspring 462 therein. Spring retainer 460 may have an open bottom and apartially open top defining a central opening 466. A top end of spring462 may engage the underside of a spring spacer 464 disposed inside thespring retainer beneath central top opening 466 configured to receivemagnetic block 402 at least partially therethrough (see, e.g. FIGS. 15and 16). The spring spacer 464 may be generally shaped as a washerhaving a diameter larger than the diameter of central opening 466 toprevent the drive extension spring 462 from being ejected out the top ofthe spring retainer 460. The bottom of magnetic block 402 may bearagainst the top side of spring spacer 464 in some positions. Liftinghead 410 may further include a stepped portion 425 formed in the topsurface 427 and/or upper bearing surface 424 which engages a bottomannular edge 429 of spring retainer 460 for locating the spring retaineron the lifting head. In one embodiment, as shown in FIGS. 9 and 10,lifting head 410 and spring retainer 460 may be disposed in the generalproximity of top end 401 of actuator shaft 404 spaced axially downwardsfrom the top end.

With continuing reference to FIGS. 9 and 10, the lower portion of thedrive rod extension (DRE) 400 includes an adapter sleeve 440 having abottom end 444 and a top end 442 attached to the bottom end 428 of theactuator tube 406. Adapter sleeve 440 has a hollow cylindrical bodywhich slidably receives actuator shaft 404 therein. In one embodiment,the bottom end 428 of the adapter sleeve 440 may be open. Actuator cap454 may be inserted through the open bottom end 428 of adapter sleeve440 to threadably engage bottom end 403 of actuator shaft 404 via afastener.

Adapter sleeve 440 includes an RCCA locking mechanism configured forreleasably coupling the sleeve to the rod cluster control assembly(RCCA) 500. In one embodiment, the locking mechanism may be a lockingelement assembly 450 comprised of a plurality of circumferentiallyspaced apart and radially moveable locking elements. The lockingelements in one exemplary configuration may be locking balls 452 whichmay be retained on an outer surface of the adapter sleeve 440 by ballretaining plates 451 spaced circumferentially about the sleeve. Thelocking balls 452 are engageable with an annular machined groove 510formed on an inside surface of a tubular mounting extension 506 risingupwards from a hub 508 of the RCCA 500 (see, e.g. FIG. 11B). The lockingballs 452 are actuated by the actuator cap 454, as further describedherein.

When the drive rod extension (DRE) 400 is mounted in the reactor vessel110, the adapter sleeves 440 of each DRE are located proximate to thebottom ends of lower guide tubes 162 in the drive rod extension supportstructure (DRESS) 160. This positions the adapter sleeve 440 toreleasably engage the rod cluster control assembly (RCCA) 500 via thelocking ball assembly 450. The locking ball assembly 450 is operable tocouple and uncouple the RCCA 500 from the DRE 400, as further describedherein.

The fuel core 116 is located at the bottom of the reactor vessel 110supported inside the core support structure 115, such as riser pipe 119.On top of the fuel core 116 is the drive rod extension support structure(DRESS) 160. The DRESS 160 is oriented such that each guide tube isaxially and vertically centered above a RCCA 500 installed in the fuelcore 116. The drive rod extensions (DRE) 130 are each positioned in theDRESS 160 and the lower portion of each DRE is seated in and looselyengaged with an RCCA 500, although not yet locked in place duringinitial assembly as evidenced in FIG. 11B showing the actuator cap 454positioned below the locking ball assembly 450 near the bottom of theadapter sleeve 440.

Control Rod Drive System Operation

An exemplary method for coupling a control rod drive mechanism (CRDM)300 to a rod cluster control assembly (RCCA) 500 will now be describedwith various reference to FIGS. 11-17 showing sequential steps in themethod or process. The drive rod extension support structure (DRESS) 160is not shown in these figures for clarity. In one embodiment, asdescribed in greater detail below, the method may be generallyaccomplished by first coupling the drive rod 130 to the top of the driverod extension (DRE) 400 which will enable the DRE to then be finallycoupled to the RCCA 500. It should be noted that the following processaddresses the coupling of a single CRDM 300 to a RCCA 500. This sameprocess, however, may be repeated for making the other CRDM-RCCAcouplings for embodiments of control rod drive system (CRDS) 100 inwhich multiple RCCAs are each individually controlled by a separatededicated CRDM.

The reactor vessel 110 is initially provided with the drive rodextension support structure (DRESS) 160 installed above the fuel core116 in the core support structure 115, in this embodiment tubular riserpipe 119. DRE 400 is preliminarily installed and inserted in the driverod extension support structure (DRESS) 160. The DRE 400 is positionedwithin the upper and lower guide tubes 161, 162. At this juncture,however, the DRE 400 is initially not operably coupled to either theRCCA 500 or the drive rod assembly (i.e. drive rod extension grappleassembly (DREGA) 200 attached to drive rod 130).

As shown in FIG. 11A, the drive rod extension (DRE) 400 is in an initialor starting vertical axial position with the top end of the actuatorshaft 404, lifting head 410, and bobbin 430 exposed and extending aboveretaining collar 170 of the drive rod extension support structure(DRESS) 160. The makes the upper portion of DRE 400 accessible to thedrive rod extension grapple assembly 200 below the top head 113 of thereactor vessel 110. In this initial position of DRE 400, the flange 416of lifting head sleeve 408 may be engaged with the retaining collar 170and the lifting head sleeve is engaged with the radially biasedretaining pins 172 of the collar.

At the bottom end of the DRE 400, the adapter sleeve 440 is positionedand inserted into, but not lockingly engaged with the tubular mountingextension of the rod cluster control assembly (RCCA) 500. Accordingly,at this initial starting position, the RCCA 500 cannot be operablyraised or lowered by CRDM 300 because the RCCA has not yet been operablycoupled and locked to the DRE 400.

To engage the DRE 400 with the RCCA 500 at the fuel core 116, the DREGA200 is first connected to the DRE in the overall coupling process. TheDREGA 200 and drive rod 130 are axially (vertically) aligned with butspaced apart from top end 401 of DRE 400 (see FIG. 11A). The CRDM 300 isoperated to lower the drive rod 130 with DREGA 200 attached theretotowards the top end 401 of DRE 400. As the DREGA 200 is lowered onto theDRE 400, the lifting pins 216 initially in a fully extended positionengage angled upper bearing surface 424 of lifting head 410 (see FIGS.4, 10, and 12A). The lifting pins 216 and lift springs 218 graduallyretract farther and farther into the DREGA housing 222 on the grapplebody 202 as DREGA 200 continues to be lowered and pushed over thelifting head 410 of DRE 400. The lifting pins 216 slidingly engage theupper bearing surface 424 moving from top to bottom of the lifting head410 (see FIG. 12B). The lift springs 218 become compressed by theretracting motion of the lifting pins 216.

When the lifting pins 216 clear and reach a position just beneath thelifting head 410, the pins return to their original fully extendedpositions inside DREGA interior chamber 212 under the inwards biasingforce of the lift springs 218 (i.e. lifting pins are in a positionslightly above that shown in FIG. 13). The DREGA 200 is now attached tothe DRE 400 and lifting pins 216 are positioned above the bobbin 430 asshown. It should be noted that DREGA 200 cannot be disengaged from DRE400 at this point with the lifting pins 216 in this axial position bymerely raising the drive rod and DREGA with the CRDM 300.

Accordingly, the method carries on by continuing to lower the DREGA 200until the electromagnet 228 in the DREGA comes into complete physicalcontact with the magnetic block 402 fastened to the top end 401 of theDRE actuator shaft 404, as shown in FIG. 13. The electromagnet 228 isthen activated (energized) from a power source. Activation of theelectromagnet 228 causes the magnetic block 402 to be releasably coupledto the electromagnet. After this magnetic coupling is completed, theDREGA 200 and drive rod 130 assembly is now fully connected to the DRE400 such that raising and lowering the drive rod using CRDM 300concomitantly raises and lowers the actuator shaft 404 of the DRE aslong as the electromagnet 228 remains energized.

In the foregoing position shown in FIG. 13, it should be noted thatdrive extension spring 462 is uncompressed. The bottom of the magneticblock 402 is positioned proximate to and may be in contact with the topof the spring retainer 460.

In order to attach the RCCA 500 remotely situated at the top of the fuelcore 116 from the CRDM 500 to the DRE 400, the actuator shaft 404 in oneembodiment needs to be pulled up to force the locking balls 452 radiallyoutwards through the adapter sleeve 440 and into the machined groove 510located in the RCCA which engages the actuator shaft with the RCCA tocomplete the coupling at the bottom of the DRE. At this point in theinstallation process, the lifting head sleeve 408 of DRE 400 is still inits initial axial starting position shown similarly in FIGS. 11A and 13,but with the DREGA 200 magnetically coupled to the DRE as shown in FIG.13. The uncoupled DRE 400 and RCCA 500 are in their respective lowermostinitial positions and at the bottom of their vertical range of travel inthe reactor vessel 110 and DRESS 160. The control rods 504 are fullyinserted in the fuel core 116. The lifting head sleeve 408 remains asyet engaged with the retaining pins 172 in retaining collar 170. Withadditional reference to FIG. 10A, the recessed annular seating surface423 of lifting head sleeve 408 is engaged with the spring biasedretaining pins 172 of retaining collar 170 which serve to releasablyhold the sleeve 408 in position during coupling of the DREGA 200 to theDRE 400. As a point of reference, it may be noted that the lifting headsleeve stop flange 416 may still rest on the top of retaining collar 170at present (see, e.g. FIG. 13) which prevents the lifting head sleeve408 from dropping any lower into the upper guide tube 161 of the DRESS160.

With the DRE 400 in the position of FIG. 13 and the foregoing magneticcoupling completed of the DREGA 200 with the DRE, the DREGA is then nextraised upwards by a first vertical distance (via the drive rod 130 usingCRDM 300) which pulls and slides the actuator shaft 404 upwards insidethe adapter sleeve 440 which remains stationary. The actuator cap 454mounted to the bottom of the actuator shaft 404 moves axially upwardswith the shaft from an unlocked position (shown, e.g. in FIG. 11B) to alocked position (shown, e.g. in FIG. 14B) forcing the locking balls 452radially outwards from the adapter sleeve 440 to engage the machinedgroove 510 inside RCCA 500. As shown in FIG. 14B, the DRE 400 is nowfully but releasably coupled at the bottom to RCCA 500 which can beraised or lowered by the CRDM 300 via the DRE 400. Accordingly, the CRDM300 has now been linked to the RCCA 500 for controlling the insertiondepth of the control rods 504 into the fuel core 116 for controlling thereactivity.

It should be noted that in the unlocked position of actuator cap 454(see, e.g. FIGS. 11B, 15B, or 16B), the larger diameter lower actuatingportion 470 of the cap with annular bearing surface 472 does not contactthe locking balls 452 which remain seated but relatively loose in theball retaining plate 451. This does not create positive lockingengagement of the locking balls 452 with the machined groove 510 on theinside of the tubular mounting extension 506 of RCCA 500 sufficient tocouple the DRE 400 to the RCCA. The reduced diameter upper portion 471of actuator cap 454 even when positioned adjacent to the locking balls452 (see, e.g. FIG. 10B) leaves an annular gap G between the cap andadapter sleeve 440 so the locking balls 452 remain loose and notpositively engaged with the machined groove 510 of the RCCA 500.

In the locked position of the actuator cap 454 (see, e.g. FIG. 14B), theannular bearing surface 472 of the larger diameter lower actuatingportion 470 of the cap is adjacent to and contacts locking balls 452.Since there is no appreciable annular gap or space between the lowerportion 470 of actuator cap 454 and adapter sleeve 440 , the annularbearing surface 472 drives the locking balls 452 outwards to engagemachined groove 510 of the RCCA tubular mounting extension 506 whichpositively couples the DRE 400 to the RCCA 500. In one embodiment, asloping transition 475 (see, e.g. FIG. 16B) may be formed between thelarger diameter lower portion 470 and reduced diameter upper portion 471of the actuator cap 454 to provide smooth sliding operation andengagement of the lower portion 470 with the locking balls 452.

After the RCCA 500 has been coupled to the CRDM 300 in the foregoingmanner, the RCCA remains in its bottom and lowermost position within thelower guide tubes 162 proximate to the top of the fuel core 116. Toprovide the ability to operationally retract the control rods 504 fromthe fuel core 116, the DREGA 200 is slightly raised further upwards ifnecessary via the CRDM 300 until the lifting pins 216 engage the bottomof lifting head 410 (as shown in FIG. 14A) if not already engaged by theDREGA-RCCA coupling process). Until the lifting pins 216 engage theunderside of lifting head 410, this initial limited upward range oftravel raises the actuator shaft 404 and DREGA 200, but not the liftinghead sleeve 408 which remains engaged with retaining collar 170 andretaining pins 172.

DREGA 200 is then further raised through a second upward verticaldistance and range of travel which pulls both the actuator shaft 404(via the magnetic coupling with the DREGA) and lifting head 410 withlifting head sleeve 408 fixed thereto upwards together simultaneously.This action disengages the lifting head sleeve 408 from the retainingpins 172 in retaining collar 170 as also shown in FIG. 14A. The DRE 400(including actuator shaft 404, lifting head sleeve, actuator tube 406,and adapter sleeve 440 shown in FIGS. 10A and 10B) and the RCCA 500coupled thereto may now be freely raised as a unit to a maximum heightwithin the reactor vessel 110 representing the fullest retractedposition of the control rods 504 from the fuel core 116 during normaloperation of the reactor vessel 110. The actuator shaft 404 and liftinghead sleeve 408 may further be alternatingly lowered and then raisedagain through a plurality of possible axial positions via operation ofthe CRDM 300 and drive rod 130.

It may be noted that the RCCA 500 fits inside and slides axially upwardand downward within the confines of the lower guide tubes 162 of theDRESS 160 which have a diameter selected to fully receive the RCCAtherein in one embodiment. The length of the lower guide tubes 162establishes the maximum vertical range of travel of the RCCA 500 andcorrespondingly the control rods 504 mounted thereto.

A method to detach the rod cluster control assembly (RCCA) 500 from thedrive rod extension (DRE) 400 and CRDM 300 for SCRAM events or otherpurposes such as opening the reactor vessel head will now be described.In one embodiment, the electromagnet 228 is first de-activated. Thisallows the actuator shaft 404 to fall or drop by a preset distancedetermined by the drive extension spring 462 and the spring spacer 464.Doing so permits the locking balls 452 to fall into the gap G created bythe reduced diameter upper portion 471 of the actuator cap 454. The RCCA500 is now disengaged from the actuator shaft 404 of drive rod extension(DRE) 400 and the CRDM 300. The foregoing falling action of the actuatorshaft 404 also re-engages the lifting head sleeve 408 with the retainingpins 172 in retaining collar 170 of the DRESS 160 (see FIG. 15A). Itshould be noted that this uncoupling action ensures that the controlrods attached to the RCCA 500 remain fully inserted into the fuel core116 which shuts down the nuclear reaction. FIGS. 14 and 15 illustratethis foregoing uncoupling sequence.

When in the foregoing position, it should be noted that the DRE 400 canalso be completely removed from the drive rod extension supportstructure (DRESS) 160 if desired by simply lifting the drive rodextension grapple assembly (DREGA) 200 via the control rod drivemechanism (CRDM) 300. Because the electromagnet 228 has beende-energized, this lifting action will disengage the lifting head sleeve408 from the retaining pins 172 in retaining collar 170 of the DRESS 160(see also FIGS. 5A and 11A).

A method for uncoupling and removing the DREGA 200 from the DRE 400(remaining in place in DRESS 160) will now be described. First, theelectromagnet 228 is deactivated (and the RCCA 500 is unlocked) in themanner already described above and shown in FIGS. 15A and 15B. Next, theDREGA 200 is pushed downwards via the CRDM 300 (and drive rod 130) toengage the bobbin 430. The lifting pins 216 initially engage angledupper bearing surface 432 which increasingly drives the pins radiallyoutwards (i.e. retracted from chamber 212) back into the DREGA 200 asthe pins advance downwards along the upper bearing surface. The liftingpins 216 reach a maximum retracted position at the apex A of the bobbin430, and then increasingly begin projecting back inwards into chamber212 of DREGA 200 again as the pins travel downwards along the angledlower bearing surface 434 (see FIG. 16A). Eventually, the lifting pins216 become fully extended beneath the bobbin 430 immediately above stopflange 416 on lifting head sleeve 408. The downward movement of DREGA200 simultaneously compresses drive extension spring 462 as shown inFIG. 16A which allows the positioning of lifting pins 216 below bobbin430 to occur. Note that a portion of magnetic block 402 has passedthrough the central opening 466 and entered spring retainer 460 tocompress the spring 462.

To complete the uncoupling of DREGA 200 from the DRE 400, the DREGA isthen raised concomitantly lifting the bobbin 430 with it via the liftingpins 216 into the lifting head 410 until the bobbin cannot move anyhigher, as shown in FIG. 17A. This occurs when the angled upper bearingsurface 432 of bobbin 430 enters cavity 426 and engages complementaryconfigured lower bearing surface 414 of lifting head 410. The bobbin 430is now nested in lifting head 410. As the DREGA 200 then continues to beraised, the lifting pins 216 will again retract outward back into DREGAhousing 222 and ride along the outside of the bobbin (angled lowerbearing surface 434) as shown in FIG. 17B. The lifting pins 216 thenengage and slide along angled upper bearing surface 424 of lifting head410 whereon the pins again increasingly begin projecting back inwardsinto chamber 212 of DREGA 200. Eventually, the lifting pins 216 becomefully extended and are free of the lifting head 410 as shown in FIG.17C. The DREGA 200 is now fully disengaged from the drive rod extension(DRE) 400 which in turn has disengaged the CRDM 300 from the DRE.

A control rod drive system according to the present disclosure providesnumerous advantages, including the following.

The length of the CRDM drive rod 130 may be limited to a relativelyshort length that is easily manufacturable. The shorter length drive rodhas the added benefits of ease of maintenance.

There is no risk of the drive rod being damaged during a SCRAM becausethe drive rod does not fall in a SCRAM event for full insertion ofcontrol rods into the fuel core to suppress the nuclear reaction as inprior known designs. In embodiments of the present invention, thecontrol rod assembly (RCCA) 500 holding the control rods is released byuncoupling the RCCA from the drive rod extension (DRE) 400 during aSCRAM. Furthermore, because the drive rod does not fall during a SCRAM,the top nozzle of the fuel assembly is not at risk for being damagedduring a SCRAM.

The complex electromechanical components in the CRDM system 100 are notsubject to the harsh environment inside of the reactor vessel becausethe CRDM 300 is mounted external to the reactor vessel.

The redundant rod ejection protection device (REPD) 140 eliminates thepotential for the drive rod 130 to be ejected from the reactor vesseldue to a CRDM housing failure.

A final advantage is that the CRDS 100 may be designed so that so thatthe CRDS will always SCRAM under gravity if the power to the CRDM 300 iscut via magnetically uncoupling the DREGA 200 from the DRE 400, asdescribed above.

Unless otherwise specified, the components described herein maygenerally be formed of a suitable material appropriate for the intendedapplication and service conditions. A suitable metal is generallypreferred for the components described herein with exception of themagnetic components. Components exposed to a corrosive or wettedenvironment may be made of a corrosion resistant metal (e.g. stainlesssteel, galvanized steel, aluminum, etc.) or coated for corrosionprotection.

While the foregoing description and drawings represent exemplaryembodiments of the present disclosure, it will be understood thatvarious additions, modifications and substitutions may be made thereinwithout departing from the spirit and scope and range of equivalents ofthe accompanying claims. In particular, it will be clear to thoseskilled in the art that the present invention may be embodied in otherforms, structures, arrangements, proportions, sizes, and with otherelements, materials, and components, without departing from the spiritor essential characteristics thereof. In addition, numerous variationsin the methods/processes described herein may be made within the scopeof the present disclosure. One skilled in the art will furtherappreciate that the embodiments may be used with many modifications ofstructure, arrangement, proportions, sizes, materials, and componentsand otherwise, used in the practice of the disclosure, which areparticularly adapted to specific environments and operative requirementswithout departing from the principles described herein. The presentlydisclosed embodiments are therefore to be considered in all respects asillustrative and not restrictive. The appended claims should beconstrued broadly, to include other variants and embodiments of thedisclosure, which may be made by those skilled in the art withoutdeparting from the scope and range of equivalents.

1-51. (canceled)
 52. A method for coupling a control rod drive mechanismto a rod cluster control assembly in a nuclear reactor vessel, themethod comprising: providing: a reactor vessel having a top head and aninterior cavity; a nuclear fuel core supported in the interior cavity; arod cluster control assembly positioned at a top of the fuel core andcomprising a plurality of control rods configured for removableinsertion the fuel core; a control rod drive mechanism mountedexternally above the reactor vessel; a drive rod assembly including adrive rod mechanically coupled to the control rod drive mechanism andextending into the interior cavity of the reactor vessel, and a grappleassembly disposed on an end of the drive rod and including anelectromagnet; lowering the drive rod assembly; contacting the drive rodassembly with a top end of a drive rod extension extending verticallybetween the rod cluster control assembly and the top head of the reactorvessel, a bottom end of the drive rod extension contacting the rodcluster control assembly in a non-locking manner; engergizing theelectromagnet to magnetically couple the drive rod assembly with thedrive rod extension; raising the drive rod assembly by a first verticaldistance; locking the bottom end of the drive rod extension with the rodcluster control assembly, wherein raising and lowering the drive rodassembly with the control rod drive mechanism raises and lowers the rodcluster control assembly for controlling the reactivity within the fuelcore.
 53. The method of claim 52, wherein the lowering step includes:engaging a plurality of radially biased lifting pins movably disposed ingrapple assembly with a lifting head disposed on the drive rodextension; retracting the lifting pins at least partially outwards intothe grapple assembly; moving the grapple assembly downwards over thelifting head; and projecting the lifting pins back inwards from thegrapple body.
 54. The method of claim 53, further comprising: engagingthe lifting pins with a bottom of the lifting head; raising the driverod assembly by a second vertical distance; and uncoupling a liftinghead sleeve on the drive rod extension from a retaining collar in thereactor vessel.
 55. The method of claim 52, further comprising:de-energizing the electromagnet; dropping the drive rod extensionwherein the drive rod assembly remains stationary; and uncoupling thebottom end of the drive rod extension from the rod cluster controlassembly.
 56. The method of claim 53, further comprising: lowering thedrive rod assembly while contacting the top end of a drive rod extensionwith the drive rod assembly; compressing a spring against the liftinghead with the drive rod assembly; engaging the lifting pins with avertically movable annular bobbin on the drive rod extension; liftingthe bobbin into engagement with the lifting head; retracting the liftingpins at least partially outwards into the grapple assembly; moving thegrapple assembly upwards over the lifting head; and projecting thelifting pins back inwards from the grapple body, wherein the drive rodassembly is uncoupled from the drive rod extension.
 57. The method ofclaim 52, wherein the locking step includes driving a plurality oflocking elements radially outwards from the drive rod extension toengage a groove formed in the rod cluster control assembly, therebylocking the rod cluster control assembly to the drive rod extension. 58.The method of claim 57, wherein the locking elements are movablydisposed in an adapter sleeve mounted on a bottom end of the drive rodextension, the adapter sleeve being partially inserted into an upwardstanding tubular mounting extension disposed on the rod cluster controlassembly.
 59. The method of claim 52, wherein the drive rod extension isslidably disposed in a guide tube of a drive rod extension supportstructure mounted between the fuel core and the top head of the reactorvessel.
 60. The method of claim 59, further comprising raising andlowering the rod cluster control assembly inside the guide tube with thecontrol rod drive mechanism.